This invention relates to semiconductor laser arrays employed in communication systems.
It is desirable to provide arrays of individual broadband laser devices fabricated on a single monolithic semiconductor structure or xe2x80x9cbarxe2x80x9d and packaged in a standard 14-pin package previously intended for single element broadband laser devices so that the array could be operated in the same manner as a single laser element. Such plug or board-level functionality equivalence, such as signal drive requirements, bias inputs, optical power feedback, etc, would simplify upgrading. Several constraints make straightforward implementation of 14-pin packages of WDM laser arrays difficult. For example, the fabrication of WDM laser arrays (such as distributed feedback [DFB] laser arrays) is facilitated when the starting III-V-type semiconductor substrate material is an n-type. Monolithically fabricating a multiplicity (e.g., 4 or 8) of diode lasers on such a substrate and singulation into a laser array xe2x80x9cbarxe2x80x9d yields a common-cathode laser array configuration. In such a common-cathode laser array configuration, it is ordinarily necessary to provide a source of current for dc laser bias and ac laser modulation input signal currents into the individual anode inputs in order to operate each laser, whether individually or in groups.
The data rates of interest using these wideband lasers are typically 1.25 Gb/s to 2.5 Gb/s or more. Ordinarily, these data rate and bandwidth restraints constrain the type of IC device technology useable to provide the modulating current input to the devices. Laser driver integrated circuits that provide high-quality modulating current signals for such lasers are typically fabricated using very high speed GaAs (or other III-V semiconductor) or SiGe NPN heterojunction bipolar transistor (HBTs) device technologies. These NPN HBT technologies offer very high transistor ft values for minimum distortion of the very high speed signal modulating current waveforms (which, at the optical communications system level, maximizes the xe2x80x9copennessxe2x80x9d of the eye diagram, implying minimal inter-symbol interference and fast signal risetimes). Unfortunately, NPN HBT devices (as well as similar very high ft n-channel FET devices fabricated in GaAs or other III-V semiconductors) are best suited for implementing current-sinking circuits rather than current sourcing circuits. The natural high-impedance (xe2x80x9ccurrent sinkxe2x80x9d-like) electrode of an NPN transistor is the collector, and in an n-channel FET the high-impedance electrode is the drain, and the dc current path is from the load device (e.g., the laser) into the collector of the NPN HBT (or drain of the n-channel FET) and out of the emitter (or source) electrode. (The electron path in the NPN HBT is from the emitter to the collector; for an n-channel FET, the electron path is from the source to the drain. Since the charge of the electron is negative, the direction of conventional positive current flow is into the collector or drain.)
It would be straightforward to connect a laser array bar with the common electrode (the laser substrate) at ac ground and to connect each of the separate laser contacts to the high impedance driver device electrode (collector or drain). Unfortunately, the preferred (n-substrate) common-cathode laser array configuration is not compatible with the preferred NPN HBT or n-channel FET driver device configurations. Although the common-cathode laser array elements would require individual (positive) current sources, these NPN HBT collector and n-channel FET drain electrodes function as (positive) current sinks (negative current sources). While the use of PNP HBTs or p-channel FET devices would circumvent this incompatibility, the poorer high-speed performance of p-channel GaAs or other III-V semiconductor FETs makes them largely unavailable as elements in standard commercial ICs, just as there is virtually no availability of PNP HBT devices in commercial III-V HBT foundry processes or in commercial SiGe HBT IC production.
What is needed is a mechanism to overcome these problems, thus allowing an inexpensive CMOS IC chip to function as the laser selection device with a common-cathode (n-substrate) diode laser array bar, and allowing its packaging in a standard 14-pin laser package with board-level functionality that closely mimics the standard single laser product.
According to the invention, an IC laser array package is provided wherein standard CMOS integrated circuit (IC) processes are used for fabricating the controller for the laser array and wherein p-channel MOSFET devices are used as switches with the controller which short the anode of the selected laser in the array (connected to the drain of the p-channel MOSFET switches) to ground. In this structure, the modulating signal from the driver input can be applied to the common cathode substrate of the laser array bar in a standard package, along with a negative dc bias current provided from the negative voltage dc bias package pin through an inductor, in the same built-in bias tee manner previously used with a standard single-laser 14-pin package. Because the p-channel MOSFETs are used only as switches, their ft values are typically not a material hindrance to the circuit operation.
High modulating signal quality is achieved so long as the xe2x80x9coffxe2x80x9d drain capacitance of the MOSFETs is low.
In addition to the p-channel MOSFET switches themselves, the CMOS wideband WDM laser array multiplexer chip also carries serial-to-parallel decoding logic that allows the selection of the desired laser or lasers for operation at any given time to be controlled through a single package pin. This serial data laser selection input is applied through one of the two available NC (xe2x80x9cno connectxe2x80x9d) pins of a standard 14-pin single-laser package. The other NC pin can be used to power the CMOS chip itself, or alternatively to share power and data clock/strobing functionality.
This configuration means that an array of wideband WDM (wavelength division multiplexing) lasers can be fabricated and mounted in a standard 14-pin single laser package configuration. Wideband (e.g., 2.5 Gb/s or more) laser modulating signals can be applied to the selected laser in a common-cathode laser array without actually routing the signal through the gate transconductance gm of the active MOSFET devices in the IC, which avoids degradation of the signal quality due to the limitations of the current gain-bandwidth product, ft, of the MOSFETs in the IC process. Avoiding the degradation in the quality of the modulating signal applied to the selected laser is thus accomplished.
The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.